The present invention relates to circuitry for controlling the firing of thyristors and more particularly to control circuitry for thyristors in high power applications incorporating protection against thyristor misfire or damage.
Thyristors have attained wide application in switching circuits for power supply systems. In this context, there has been demonstrated a need for circuitry capable of gating the thyristor in power applications up to several thousand volts and at the same time protecting the thyristor from misfiring and/or being damaged. It is desirable that such gating circuitry embody certain functions; viz., isolation of control circuitry from the thyristor, high gate drive capability and protection of the thyristor from misfiring and or being damaged.
A number of techniques for isolating a thyristor from its control circuitry have been developed. Usually this is accomplished by utilizing some form of a transformer, such as a pulse transformer, for thyristor gating. The disadvantage of using transformers for thyristor gating are the transformers have relatively high capacitive coupling between the windings unless extensive shielding is employed. This capacitive coupling can allow noise current to flow from the power line into the control circuits if high rate of changes of voltages exist between the power line and the control circuits. These currents will flow in accordance with the relationship: EQU i=c(dv/dt)
where
i=current flowing into control circuit PA1 c=capacitance between windings of thyristor gating transformer PA1 dv/dt=rate of change of voltage between the power line and the control circuit
This current will contain high frequency components and can produce voltage drops along the control circuit common (control circuit power return or reference) of sufficient amplitude as to cause a malfunction of the control logic. This will usually result in a misfiring of thyristors.
Further, when transformers are employed for thyristor gating, a separate power supply must be employed to provide power for the "high gate drive" circuitry.
Certain prior art methods of optical isolation employ photo-SCRs to optically couple the control circuitry to the thyristor and, generally, these function well. These devices comprise a light emitting diode (LED) and an SCR that is "gated-on" optically when current is passed through the diode causing light to be emitted thus actuating the SCR. This device is made as a unit wherein the package contains the light emitting diode and light sensitive SCR, and has a very low capacitive coupling between the input LED and the output light sensitive SCR. Because of the low capacitive coupling between the input and output, typically 2.0 picofarads, it would appear that this device would have the capability of giving the desired isolation between power line and control circuit as well as the capability of providing the high gate drive. However, it has been found that this device has at least one very serious limitation which precludes its use in many applications. The light sensitive SCR possesses a dv/dt (rate of change of voltage from anode to cathode) rating that is largely a function of the resistance used to terminate the gate to cathode junction. The current required to flow through the LED to cause the SCR to be "gated-on" is also a function of the resistance terminating the gate to cathode junction. Usually, the resistance for terminating this junction cannot be satisfactorily chosen so that the SCR will have an adequate dv/dt rating and at the same time require tolerable values of current through the LED for gating the SCR. Another disadvantage of this type device is that the SCR is only available with a peak inverse voltage rating to 400 volts. This would limit the usage of the device to systems where the voltage across the thyristor to be "gated-on" has a maximum RMS value of 240 V.A.C. the device would therefore not be useful on 440 to 480 volt systems.
As for providing a high gate drive capability, a darlington transistor pair is well suited to perform this function. However, in high power applications, it is expedient to provide circuitry for increasing the off-state blocking voltage capability of a darlington configuration since the voltage across the configuration can go from a very low value to a very high value (possible several thousand volts) when the thyristor is switched from a conducting to a non-conducting state. And, finally, means for protecting the thyristor from damage or misfiring is advisable in high power applications high rate of rise of voltage across the thyristor.